The present invention relates to a method of imparting increased dezincification-resistance to brass.
Brasses are alloys of copper and zinc. When containing up to 37% (in this specification % is understood to mean percent by weight) of zinc the alloys are so-called .alpha.-alloys, which are solid solutions built up of homogeneous crystals with a face-centred cubic lattice. Such an .alpha.-brass possesses good cold-working properties, it can be subjected to rolling, stamping and upsetting in the cold condition, but it is rather more difficult in hot working, especially if it contains lead, which is often added to increase its machining properties.
One of the causes of the wide-spread application of brass is the good resistance of this type of alloy to corrosion in air and water, but under certain circumstances, especially if brass is exposed to soft, chloride-containing water and particularly in hot surroundings, brass will corrode in a special way called dezincification, which manifests itself in a dissolution of the zinc content, while the dissolved copper is reprecipitated as a spongy, porous mass without strength and with a tendency to develop through perforations, so that e.g. brass fittings for water piping may become leaky under pressure and unfit for further use, when dezincification corrosion sets in.
It is known that dezincification of .alpha.-brass can be prevented, if small quantities of arsenic, antimony or phosphorus in the range of 0.01% or upwards are added to the alloy. On account of the great ductility of the .alpha.-brasses in the cold condition they are used particularly for the production of sheet, strip, tube, and wire for cold upsetting purposes, e.g. for nails, screws and the like.
At increased zinc content, i.e. from approx. 37% another form of crystal will appear, the so-called .beta.-crystal, which has a body-centered cubic lattice.This .beta.-crystal is distinguished by a considerably better ductility in the hot condition than in the cold, and alloys containing .beta.-crystals are therefore particularly useful for hot-forging and hot-extrusion of sections and rods and also more useful for pressure die-casting, chill casting and sand casting than .alpha.-brass. This type of alloy may contain several percent of lead for the benefit of its machining properties without adversely affecting its hot working properties.
This type of alloy usually contains 63 - 56% of copper, the remainder being zinc and lead. As regards corrosion these alloys are resistant to the influences of water and air, too, but when subjected to dezincification-promoting surroundings it is not posible to prevent dezincification by the addition of small amounts of antimony, phosphorus or arsenic on account of the content of .beta.-crystals, which increases with decreasing copper content. For a brass containing e.g. 58% of copper the .beta.-quantity will be about 30%, and even if the said dezincification-preventing agents have been added to a brass with such content, it will corrode vigorously under conditions favouring dezincification, because the 30% .beta.-phase forms a coherent structure in the alloy and cannot be protected against dezincification. Even at small quantities of .beta.-phase this phase forms a sort of network through the structure, so that alloys with small .beta. -quantities cannot be protected against dezincification by means of the said alloy additions.